The present invention relates to a power steering system for turning steerable vehicle wheels and, more specifically, to a power steering control valve used in the power steering system.
A known power steering system includes inner and outer valve members which are rotatable relative to each other to port fluid to a vehicle steering motor. A torsion bar spring is connected with the inner and outer valve members to resist relative rotation between the inner and outer valve members and to urge the inner and outer valve members to their initial positions when rotational force is no longer applied to a steering wheel and the inner valve member. A power steering control valve having this construction is disclosed in U.S. Pat. No. 5,230,273. Another known power steering control valve is disclosed in U.S. Pat. No. 4,339,986.
It has previously been suggested that the resistance provided by a torsion bar spring to relative rotation between inner and outer valve members could be varied by varying the effective length of the torsion bar spring. Thus, in U.S. Pat. No. 4,481,866, an anchor nut extends between a torsion bar spring and an inner valve member. By moving the anchor nut along the torsion bar spring, the effective length of the torsion bar spring is varied. In U.S. Pat. No. 3,930,554, a plunger is moved into engagement with a recess in a central portion of a torsion bar spring to decrease the effective length of the torsion bar spring. Decreasing the effective length of the torsion bar springs increases the resistance of the torsion bar spring to relative rotation between the inner and outer valve members.